1. Field of the Invention
The present invention relates to a method for extending the longevity of an electrical power cable. More particularly, the invention relates to an improved method for restoring the dielectric properties of a very long or obstructed in-service electrical power cable section wherein a dielectric enhancement fluid is injected into the interstitial void volume of the cable.
2. Description of the Related Art
In a well known method for restoring the dielectric properties of an in-service electrical power cable, a liquid composition comprising at least one organoalkoxysilane is introduced (injected) into the interstitial void volume associated with the geometry of the stranded conductor of the cable. Over an extended period, the organoalkoxysilane diffuses radially through the polymeric insulation jacket of the cable and fills microscopic defects (trees) therein. Ideally, the organoalkoxysilane hydrolyzes in the presence of adventitious water, which diffuses in from the environment, and subsequently condenses within these defects, thereby augmenting the old cable's dielectric performance. These reactions are facilitated by a catalyst included in the composition. This, in turn, results in the formation of oligomeric siloxane species which are not subject to eventual loss due to continued diffusion, and the above commercially practiced method thus offers a distinct advantage over the use of non-condensable fluids (e.g., see U.S. Pat. No. 4,766,011). Unfortunately, as desirable as the oligomerization of organoalkoxysilanes within the insulation jacket is, its condensation within the strand interstices results in increased viscosity and reduced flow rate of the treatment fluid. As a practical matter, flow can even stall entirely if this viscosity becomes too high.
Prior to the advent of the above method, strand desiccation methods were employed in an effort to restore the dielectric strength of old power cables. In this case, a dry fluid such as air, nitrogen, an alcohol or a glycol is injected into the interstitial void volume to remove water therefrom (e.g., see U.S. Pat. Nos. 4,372,988 and 4,545,133). Such a drying step has also been utilized just prior to injecting the organoalkoxysilane in the above described method to prevent premature condensation of the organoalkoxysilane due to reaction with water initially present in the interstitial void volume. But even a combination of these steps (i.e., first drying, then injecting with organoalkoxysilane) cannot guarantee that premature condensation will not occur in the interstitial volume when the organoalkoxysilane injection step takes a very long time since water still residing within the conductor shield and insulation jacket, as well as additional water from the cable's exterior, quickly diffuses back into the interstitial void volume when strand desiccation is suspended and flow of the organoalkoxysilane commences (see Bertini, “Molecular Thermodynamics of Water in Direct-buried Power Cables”, Electrical Insulation Magazine, November/December 2006). Therefore, the catalyzed organoalkoxysilane composition is again exposed to water and viscosity quickly increases in a reinforcing cycle, resulting in the aforementioned flow problem (i.e., water enters void volume→more condensation of organoalkoxysilanes increases viscosity→reduced flow rate→even more water enters void volume, etc.).
Such a long injection time is necessitated when treating very long cable sections, such as submarine cables having a length greater than about 1,000 meters. Also, cables having significantly obstructed strand geometry relative to round strand design specifications often require excessive injection times. Examples of such obstructions of the cable's interior include conductor corrosion, strand compaction, and strand compression. Furthermore, the required injection time can also be excessive when the initial viscosity of the organoalkoxysilane is too high. Finally, if an obstructed or very long cable has to be injected at relatively low pressures, this step can also take too much time. For example, a cable having a relatively weak insulation jacket material, such as ethylene-propylene rubber (EPR), can be injected only at considerably lower pressures than those possible for polyethylene (PE) insulation. Similarly, a cable having a relatively thin insulation jacket might not be able to withstand the higher injection pressures. Of course, more than one of these cumulative factors can be associated with a given cable section, resulting in an even greater increase of the required injection time. Thus, many cables currently in service, whether due to excessive length, internal obstruction or materials of construction, cannot readily be rejuvenated by the aforementioned injection of a catalyzed organoalkoxysilane, and therefore have to be replaced when they fail.